There already exist, according to the prior art, a number of electrochemical or electrolytic processes for the decontamination of water.
First, according to U.S. Pat. No. 3,766,034, there is a process to eliminate ionic impurities from the water. The impurities include sulfates, phosphates and chlorides. According to this process the water to be purified is passed through an electrolytic cell containing an anode and a cathode and containing metallic particles (Pb or Al) covered by an ion-permeable resin membrane. The reference indicates that during the passage of current between the electrodes, the particles of metal react with the impurities and the resin membrane containing same to form products which are insoluble in water and which are retained in the cell.
The British Pat. No. 1,497,134 describes a process for the sterilization of water by anodic oxidation according to which the water is circulated through a cell containing an anode and a cathode each connected to a power source. The cell also contains, situated between said anode and cathode, a plurality of auxiliary metal electrodes which are resistant to oxidation and electrically isolated from one another as well as from the anode and the cathode. The effectiveness of such an arrangement arises from the very small distance maintained between the electrodes for the passage of water. The reference also indicates that when such an arrangement is used, it is perhaps desirable to prevent the influence of the hydrogen at the cathode from disturbing the oxidation caused by oxygen formed at the anode, by covering the cathode with a porous nonconductor.
In French Pat. No. 2,311,760 and U.S. Pat. No. 4,072,596, a process is disclosed for the decontamination of water by the application of an alternating current having a frequency of 0.5 to 800 Hz. The apparatus used to carry out this process includes a container divided into a succession of several compartments through which the water to be purified goes. The compartments connected to electrodes connected to a current source contain particle beds of diverse materials, such as metal oxide catalysts (preferably oxidation catalysts), for example oxides of tin, lead, bismuth, chromium, antimony, molybdenum and others. These catalysts can be deposited on a porous support such as alumina. The particles used according to the reference can also include diverse materials such as graphite, carbon, synthetic polymers (e.g. nylon, polyurethane), ceramics, perlite, etc. The particles of carbon or graphite are mentioned because of their conductivity, which augments the efficiency of the oxidation and contributes to the formation of "minicells" and equally facilitates the removal of impurities by absorption. In order to improve the efficiency, the apparatus is provided with refrigeration means.
French Pat. No. 2,244,842 discloses an apparatus to electrochemically treat waste water. This apparatus includes a tank containing planar electrodes of a perforated conductive material and a fluidized bed of nonconductive particles. The apparatus is used principally to eliminate metals dissolved in industrial waste water, for example copper.
French Pat. No. 2,318,115 and U.S. Pat. No. 4,048,030 disclose an apparatus similar to that previously discussed designed especially for the purification and sterilization of water. The apparatus comprises essentially a closed vessel having an inlet below and an outlet above for the water, and comprises at least two electrodes connected respectively to the positive and negative terminals of a direct source of electric current. The vessel also contains free particles having a density greater than that of the treated water so that the medium which contains the particles is at the bottom of the vessel. These particles are agitated by the movement of water current traversing the cell, so that a turbulent layer of particles is formed. These particles surround the electrode and their movement permanently maintain the electrodes clean by mechanical action. When the process is carried out in such an apparatus to electrolyze water containing for example 3 ppm chloride, without using an electrolysis membrane between the electrodes, there is no formation of chlorine, but only the liberation of oxygen at the anode. On the contrary, when such a membrane is used, the concentration of chloride is augmented in the interior of the anode compartment so as to be sufficient to form free chlorine. In the absence of chloride ions, other ions, for example carbonate or sulfate, present in the water, are concentrated in the anode compartment formed by the membrane and are oxidized to form persulfate or percarbonate, which equally constitute excellent oxidizing agents. All of the organic acids contained in the water can be oxidized in the same fashion.
French Pat. No. 2,215,399 and U.S. Pat. No. 3,725,226 disclose an electrochemical process using alternating current for the decontamination of polluted water contaminated with pathogenic agents. Both references refer to an earlier technique in which an aqueous medium containing the germs is subjected to a potential insufficient to provoke the electrolysis of the water, but sufficient to cause the transfer of current between the anode and the pathogenic agents to be destroyed. At all times according to these references, these earlier techniques suffer from an important deficiency which amounts to fouling of the electrodes, requiring periodic cleanings. As a consequence both references recommend the application in the case of the earlier technique of an alternating current, rather than a direct current, where the potential applied during the anodic phase is less than that necessary for the production of oxygen by electrolysis, even though concerning the cathode phase, the potential is sufficient so that hydrogen would be formed and liberated as a gas to thereby clean the electrode.
French Pat. No. 2,316,196 discloses an apparatus for the purification of waste water by anodic oxidation. This apparatus includes a cathode and an anode, and a mass of conductive particles placed in contact with the anode by the displacement of the water to be purified. The particles are positively charged and contribute to the efficient distribution of the oxidative effect throughout the liquid to be purified.
French Pat. No. 2,288,060 concerns an electrochemical process for the sterilization of water and its purification, especially water containing toxic residues of organic agents, in an implosion medium (of the order of 100 to 6000 V) capable of creating electric discharges between the electrodes.
U.S. Pat. No. 4,048,044 discloses an apparatus for the decontamination of water by anodic oxidation. The anode in this apparatus is silver, which besides producing oxygen at its surface, also has the property of anodically dissolving itself to liberate silver ion which augments the ability of the liberated oxygen to act as a bactericide.
The prior art techniques which have been described, though interesting and varied, all suffer from disadvantages which are as follows:
The complexity of the apparatus and the media which must be used (for example French Pat. No. 2,311,760) are often disproportionate with the results obtained. The efficiency is mediocre and the need for electrical energy to treat one cubic meter of water is relatively high. The narrowness of the range of application, that is certain techniques can be applied to a process for the decontamination of water containing germs but not to the elimination of chemical pollutants or heavy metals, while in the case of other prior art processes, the reverse situation is true, poses an additional problem.